If you use the same lens on the both cameras, you would of course would will have a smaller field of view on the 7D- that's why it is called a crop camera. However you can crop the 6D to the same size FOV as you get with the 7D. When you do this, do you see more detail in the 7D image? Some people feel that a crop camera gives the equivalent of a longer focal length lens because of its narrower FOV, but you can always crop the FF image to equal the 6D FOV. The images are then the same size and the DOF is the same. However, the pixel size on the 7D is smaller, so in principle you should be able to see more detail with it; it's the film equivalent of using finer grain film (though I can't remember anyone saying they use fine grain film for a longer reach). So, do you see a significant gain in detail with the "finer-grained 7D than with the 6D. If you don't than you really don't acieve any longer reach with it.

As an example, with the same lens on both my T2i and my 5DIII, I see more detail in the T2i image jus as one might expect, and thus I could say that has a longer reach with the same lens. I would prefer to say it has higher angular resolutionbecause of the smaller pixels, but that may be just getting technical. To be sure, you won't achieve higher resolution with smaller pixels if the lens isn't up to it, if the focus isn't accurate, and if camera shake has no affect on resolution.